How to make a 3gpp file usable for the movie clip editor ?
This question comes up pretty high in searches so I thought I'd add what worked for me - converting it to mp4 with ffmpeg:

ffmpeg -i originalvid.3gp -q:v 0 -q:a 0 -strict -2 targetvid.mp4

This keeps the video (q:v 0) and audio (q:a 0) at the maximum possible quality.